[WIP] Darwin: FSWatch API

[kristate]

This is the Darwin Branch of #1294 ;

Highlights include new platform agnostic API.

[kristate]

@andrewrk hey, is this a bug?

[andrewrk]

I'm still thinking about this one. We have to decide whether name resolution is lazy or not. This is actually related to #21 and I'll use the insight from implementing that issue when figuring out how to deal with this situation.

[kristate]

I think we are in a pretty good place.

pwritev doesn't exist on darwin, but calling pwrite consecutively is recommended.

https://github.com/kristate/zig/blob/2d064bae9f8ca4d263ae58b04b484fa1a8314190/std/event/loop.zig#L676-L702

[andrewrk]

I think that we will use FSEvents on MacOS eventually. However, a pure kqueue solution (with an fd per file) will still be necessary on some of the BSDs. So we may as well implement that, because it's simpler and pushes off #1349 to solve later.

[kristate]

@andrewrk I agree about kqueue.
macOS is almost done, but we should also start to support BSD.

We should eventually have these implementations for FS watching:

• FSEvents for Darwin
• File Events Notification for Solaris
• kqueue for the BSDs
• inotify for linux (you have this already)
• ReadDirectoryChangesW on Windows

I also found this list of limitations from the @emcrisostomo's fswatch project:

• The FSEvents monitor, available only on OS X, has no known limitations,
  and scales very well with the number of files being observed.

• The File Events Notification monitor, available on Solaris kernels and
  its derivatives, has no known limitations.

• The kqueue monitor, available on any *BSD system featuring kqueue,
  requires a file descriptor to be opened for every file being watched. As a
  result, this monitor scales badly with the number of files being observed,
  and may begin to misbehave as soon as the fswatch process runs out of file
  descriptors. In this case, fswatch dumps one error on standard error for
  every file that cannot be opened.

• The inotify monitor, available on Linux since kernel 2.6.13, may suffer
  a queue overflow if events are generated faster than they are read from the
  queue. In any case, the application is guaranteed to receive an overflow
  notification which can be handled to gracefully recover. fswatch
  currently throws an exception if a queue overflow occurs. Future versions
  will handle the overflow by emitting proper notifications.

• The Windows monitor can only establish a watch directories, not files.
  To watch a file, its parent directory must be watched in order to receive
  change events for all the directory's children, recursively at any depth.
  Optionally, change events can be filtered to include only changes to the
  desired file.

[andrewrk]

I think the biggest challenge we face in order to support more target platforms is infrastructure - we absolutely must have CI building and testing for all supported platforms. See #835. Once we have automated testing set up for one of the BSDs, supporting it will be a piece of cake.

[andrewrk]

Let's solve #1349 before we add any dependencies on frameworks to the standard library. We can re-evaluate using FSWatch later.